Gas turbines are widely used in industrial and commercial operations. A typical gas turbine includes an inlet section, a compressor section, a combustion section, a turbine section, and an exhaust section. The inlet section cleans and conditions a working fluid (e.g., air) and supplies the working fluid to the compressor section. The compressor section increases the pressure of the working fluid and supplies a compressed working fluid to the combustion section. The combustion section mixes fuel with the compressed working fluid and ignites the mixture to generate combustion gases having a high temperature and pressure. The fuel supplied to the combustion section may be a liquid fuel, a gaseous fuel, or a combination of liquid and gaseous fuels. The combustion gases flow to the turbine section where they expand to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity.
The combustion gases exiting the turbine section as exhaust gases generally have considerable residual heat that may be extracted prior to discharge to the environment to further enhance the overall efficiency of the gas turbine. In a combined cycle gas turbine, the exhaust section may include a heat recovery steam generator or other heat exchanger to extract additional heat energy from the exhaust gases to produce steam which may in turn be used to generate power. In addition, the exhaust gases flowing through the exhaust section may be diverted to a fuel heat exchanger to heat the fuel supplied to the combustion section to enhance the efficiency of the combustion and reduce undesirable emissions of nitrous oxides (NOx).
The temperature of the exhaust gases may vary depending on the various operating conditions of the combined cycle gas turbine. For example, during initial startup of the gas turbine, the temperature of the exhaust gases may not be high enough to supply heat to both the fuel heat exchanger and the heat recovery steam generator. Alternately, during normal gas turbine operations, the heat recovery steam generator may need to be isolated from exposure to the exhaust gases, for example, to support scheduled or unplanned maintenance or repairs to the heat recovery steam generator. As a result, a system and method for heating the fuel using the exhaust gases over a wide range of operating conditions would be useful.